Field of the Invention
The present invention relates to a substrate holding apparatus for holding a substrate to be polished and pressing the substrate against a polishing pad (polishing surface), and more particularly to a substrate holding apparatus for holding a substrate such as a semiconductor wafer in a polishing apparatus for polishing and planarizing the substrate. Further, the present invention relates to a polishing apparatus having such substrate holding apparatus.
Description of the Related Art
In recent years, high integration and high density in semiconductor device demands smaller and smaller wiring patterns or interconnections and also more and more interconnection layers. Multilayer interconnections in smaller circuits result in greater steps which reflect surface irregularities on lower interconnection layers. An increase in the number of interconnection layers makes film coating performance (step coverage) poor over stepped configurations of thin films. Therefore, better multilayer interconnections need to have the improved step coverage and proper surface planarization. Further, since the depth of focus of a photolithographic optical system is smaller with miniaturization of a photolithographic process, a surface of the semiconductor device needs to be planarized such that irregular steps on the surface of the semiconductor device will fall within the depth of focus.
Thus, in a manufacturing process of a semiconductor device, it increasingly becomes important to planarize a surface of the semiconductor device. One of the most important planarizing technologies is chemical mechanical polishing (CMP). In the chemical mechanical polishing, while a polishing liquid containing abrasive particles such as silica (SiO2) therein is supplied onto a polishing surface such as a polishing pad, a substrate such as a semiconductor wafer is brought into sliding contact with the polishing surface and polished using the polishing apparatus.
The polishing apparatus which performs the above-mentioned CMP process includes a polishing table having a polishing surface formed by a polishing pad, and a substrate holding apparatus, which is referred to as a top ring or a polishing head, for holding a substrate such as a semiconductor wafer. When the substrate is polished with such a polishing apparatus, the substrate is held and pressed against the polishing surface under a predetermined pressure by the substrate holding apparatus. At this time, the polishing table and the substrate holding apparatus are moved relative to each other to bring the substrate into sliding contact with the polishing surface, so that the surface of the substrate is polished to a flat mirror finish.
If a relative pressing force produced between the substrate and the polishing surface of the polishing pad is not uniform over the entire surface of the substrate, then the substrate is insufficiently or excessively polished depending on the pressing force applied to each area of the substrate. Therefore, it has been attempted that a holding surface of the substrate holding apparatus is formed by an elastic membrane of an elastic material such as rubber, and a plurality of pressure chambers to which a pressurized fluid is supplied are formed at the reverse side of the elastic membrane and a fluid pressure such as air pressure is applied to the pressure chambers to uniform the pressing force applied to the substrate over the entire surface of the substrate.
If transferring of the substrate to be polished to the substrate holding apparatus and receiving of the polished substrate from the substrate holding apparatus are conducted directly by a transportation apparatus such as a robot, there is a risk of transfer error caused by variation in transfer accuracy of the substrate holding apparatus and the transportation apparatus. Therefore, a substrate transfer unit, which is referred to as a pusher, is provided at a position where the substrate is transferred to the substrate holding apparatus or a position where the substrate is transferred from the substrate holding apparatus. The substrate transfer unit is an apparatus which has a function for temporarily placing the substrate transferred by the transportation apparatus such as a robot thereon and then lifting and transferring the substrate to the substrate holding apparatus such as a top ring which has moved above the substrate transfer unit, and a function for transferring the substrate received from the substrate holding apparatus to the transportation apparatus such as a robot.
When the substrate holding apparatus such as a top ring or a polishing head transfers the substrate such as a semiconductor wafer to the pusher (substrate transfer unit), a pressurized fluid (gas, liquid, or mixed fluid of gas and liquid) is supplied into a fluid passage provided in the top ring to push the substrate out of the top ring, thus releasing the substrate from the top ring. At this time, a certain gap is provided between the top ring and the pusher, and thus the substrate falls by a distance of the gap when it is released from the top ring, and the fallen substrate is received by the pusher.
A release nozzle disclosed in Japanese laid-open patent publication No. 2005-123485 or the like, having been used to reduce stress applied to the substrate when the substrate is released from the top ring, can be thought to be alternative. The release nozzle serves as an assisting mechanism for assisting the release of the substrate from the top ring by ejecting a pressurized fluid between the rear surface of the substrate and the membrane. In this case, the substrate is pushed out downwardly from the bottom surface of a retainer ring to remove the peripheral portion of the substrate from the membrane, and then the pressurized fluid is ejected between the peripheral portion of the substrate and the membrane. Therefore, when the substrate is released from the top ring, it is necessary to inflate the membrane by pressurizing the membrane, as disclosed in the paragraph [0084] of Japanese laid-open patent publication No. 2005-123485. The release nozzle is also disclosed in U.S. Pat. No. 7,044,832. As disclosed in this U.S. patent publication, when the substrate is released, a bladder is inflated (pressurized), and then a shower is sprayed in a state in which the edge portion of the substrate is separated from the bladder (see the 6th to 15th lines of the column 10 and FIG. 2A). Specifically, in both of the above publications, the membrane is inflated to separate the edge portion of the substrate from the membrane, and a shower is sprayed into the gap. However, when the membrane in these publications is pressurized and inflated as suggested, locally varied downforce is applied to the substrate. Accordingly, stress tends to be applied to the substrate locally in accordance with inflation of the membrane, and fine interconnections formed on the substrate are broken or the substrate itself is damaged at the worst.
In contrast thereto, as a method for preventing the membrane from being inflated excessively when the substrate is released from the top ring, Japanese laid-open patent publication No. 2010-46756 discloses that at least one of plural pressure chambers is pressurized and at least one of the plural pressure chambers is depressurized to create a vacuum therein when the substrate is released from the top ring. However, it takes time to create a vacuum in the pressure chamber, resulting in poor responsiveness. Accordingly, it takes time to release (remove) the substrate. Further, when the pressure chamber is depressurized to create a vacuum therein, a local area of the substrate is pulled to increase the amount of deformation of the substrate.